Heavy duty rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam and the like. Such bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the body at its lower end. In use the drill string and bit body are rotated in the bore hole and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled. As such a rock bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a rock bit in such severe environments is in the order of 20 to 200 hours for bits in sizes of about 61/2 to 121/4 inch diameter at depths of about 5000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical.
When a rock bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation. Prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or severe wear of the journal bearings on which the cutter cones are mounted. These bearings are subject to very high pressure drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling as well as elevated temperatures in the formation being drilled. Considerable development work has been conducted over the years to produce bearing structures and employ materials that minimize wear and failure of such bearings.
The journal bearings are lubricated with grease adapted to such severe conditions. Such lubricants are a critical element in the life of a rock bit. A successful grease should have a useful life longer than other elements of the rock bit so that premature failures of bearings do not unduly limit drilling. Failure of lubrication can be detected by generation of elevated pressure in the bit, evidence of which can often be found upon examination of a used bit. The high pressure is generated due to decomposition of oil in the grease with consequent generation of gas when lubrication is deficient and a bearing overheats due to friction. Lubrication failure can be attributed to misfit of bearings, or seal failure as well as problems with a grease.
Pressure and temperature conditions in a rock bit can vary with the time as the rock bit is used. For example, when a "joint" of pipe is added to the drill string, weight on the bit can be relieved and slight flexing can occur. Such variations can result in "pumping" of the grease through seals, leading to loss of grease or introduction of foreign materials such as drilling mud that can damage bearing surfaces.
It is therefore desirable to provide a grease for lubricating rock bits that has a long useful life, does not generate substantial internal pressure in the bit and protects metal bearing surfaces from premature wear or failure.